This invention relates to a system for measuring minute quantities of solid materials and, more particularly, to a system for measuring minute concentrations of dissolved solids in liquids.
In the manufacture of semiconductor chips, super pure water and other solvents are needed. For example, in the manufacture of very large scale integrated (VLSI) circuits, the surface of the semiconductor wafer which is to become the VLSI circuit must be repeatedly washed or cleansed. Very pure water is used to wash the surface and any nonvolatile residue dissolved in the water will have a tendency to remain on the surface of the wafer when the ultrapure water has evaporated. Since minimal traces of residue material on the surface of the semiconductor wafer can cause defects in the resulting semiconductor device, it is imperative to use very pure water of the highest quality to prevent or eliminate possible defects. There is also a need for very pure water in other industries, such as in the pharmaceutical industry and in electric power generating. Water for pharmaceutical injectibles must be free from bacterial and pyrogens if it is not to cause pathological effects when injected into the human body. The electric power industry requires ultrapure water for high pressure steam generation to drive turbine generators because impurities in the water can be deposited on turbine blades and cause unbalancing of the turbine or cause corrosion. Accordingly, there is a need to be able to measure minute quantities of dissolved solvents in liquids to ensure that the liquid has the requisite purity for the application.
One effective system for measuring minute concentrations of dissolved solids and liquids in use prior to the present invention involves atomizing the liquid into droplets 10 to 100 microns in diameter, then drying the droplets to a residue in the form of a particle approximately spherical in shape. The size of the particle is then measured by passing the particle through a light beam to scatter light from the beam to photodetectors or photodetectors. The amplitude of the resulting pulse produced by the photodetector will provide a measurement of the diameter of the particle. If the size of the original droplet is known, the concentration of the dissolved solid in the liquid from which the droplet was formed can be determined.
The concentration of the solid in the liquid is related to the diameter of the residue particle and the droplet diameter in accordance with the following equation: EQU C=(d/D).sup.3 ( 1)
in which C is the concentration, d is the (1) residue particle diameter and D is the liquid droplet diameter. By making the liquid droplet bigger, a more minute concentration can be detected. However, droplets larger than 100 microns cannot be easily dried. As a practical matter to ensure reliability of the measurement, droplet sizes of around 50 microns are employed. If the concentration of the liquid were 1 part per million and the original droplet size were 50 microns, then the diameter of the residue particle would be about 0.5 microns which corresponds to the sensitivity of some relatively inexpensive particle detectors. Some higher quality particle detectors have a sensitivity to detect particles 0.3 microns in size. Assuming a 50 micron droplet since, such a particle detector can detect a concentration of (0.3/50).sup.3 or about 200 parts per billion. This sufficient sensitivity is not sufficient for VLSI circuits and some other applications. State of the art particle detectors can detect smaller particles and have sufficient sensitivity for VLSI application, but such particle detectors are many times for expensive than the particle detectors with a sensitivity to detect 0.3 micron particles.